The present invention is directed to an apparatus and system for measuring the constituent concentration value present in a liquid. This invention is applicable to sensing a hematocrit (Hct) level of blood in an extracorporeal circuit.
Renal Replacement Therapy (RRT) has evolved from the long, slow hemodialysis treatment regime of the 1960's to a diverse set of therapy options, the vast majority of which employ high permeability membrane devices and ultrafiltration control systems. Biologic kidneys remove metabolic waste products, other toxins, and excess water. They also maintain electrolyte balance and produce several hormones for a human or other mammalian body. An artificial kidney, also called a hemodialyzer or dialyzer, and attendant equipment and supplies are designed to replace the blood-cleansing functions of the biologic kidney. At the center of artificial kidney design is a semipermeable filter membrane that allows passage of water, electrolytes, and solute toxins to be removed from the blood. The membrane retains in the blood, the blood cells, plasma proteins and other larger elements of the blood.
Over the last 15 years, RRT systems have evolved into a subset of treatment alternatives that are tailored to individual patient needs. They include ultrafiltration, hemodialysis, hemofiltration, and hemodiafiltration, all of which are delivered in a renal care environment, as well as hemoconcentration, which is typically delivered in open heart surgery. RRT may be performed either intermittently or continuously, in the acute or chronic renal setting, depending on the individual patient's needs.
Ultrafiltration involves the removal of excess fluid from the patient's blood by employing a pressure gradient across a semipermeable membrane of a high permeability hemofilter or dialyzer. For example, removal of excess fluid occurs in hemoconcentration at the conclusion of cardiopulmonary bypass surgery. Hemodialysis involves the removal of toxins from the patient's blood by employing diffusive transport through the semipermeable membrane, and requires an electrolyte solution (dialysate) flowing on the opposite side of the membrane to create a concentration gradient. A goal of dialysis is the removal of waste, toxic substances, and/or excess water from the patients' blood. Dialysis patients require removal of excess water from their blood because they lack the ability to rid their bodies of fluid through the normal urinary function.
One of the potential risks to health associated with RRT is hypotension, which is an abnormal decrease in the patient's blood pressure. An abnormally high or uncontrolled ultrafiltration rate may result in hypovolemic shock, hypotension, or both. If too much water is removed from the patient's blood, such as might occur if the ultrafiltration rate is too high or uncontrolled, the patient could suffer hypotension and/or go into hypovolemic shock. Accordingly, RRT treatments must be controlled to prevent hypotension.
Alternatively, a patient may experience fluid overload in his blood, as a result of fluid infusion therapy or hyperalimentation therapy. Certain kinds of RRT machine failures may result in a blood fluid gain rather than fluid loss. Specifically, inverse ultrafiltration may result in unintended weight gain of a patient and is potentially hazardous. Uncontrolled infusion of fluid into the patient could result in fluid overload, with the most serious acute complication being pulmonary edema. These risks are similar in all acute and chronic renal replacement therapies (ultrafiltration, hemodialysis, hemofiltration, hemodiafiltration, hemoconcentration). Monitoring patients to detect excessive fluid loss is needed to avoid hypotension.
Rapid reduction in plasma or blood volume due to excessive ultrafiltration of water from blood may cause a patient to exhibit one or more of the following symptoms: hypovolemia-hypotension, diaphoresis, cramps, nausea, or vomiting. During treatment, plasma volume in the patient's blood would theoretically remain constant if the plasma refilling rate equaled the UF (ultrafiltration) rate and thus the hematocrit (Hct) would remain constant. However, refilling of the plasma is often not completed during a RRT session. The delay in refilling the plasma can lead to insufficient blood volume in a patient.
There appears to be a “critical” blood volume value below which patients begin to have problems associated with hypovolemia (abnormally decreased blood volume). Fluid replenishing rate is the rate at which the fluid (water and electrolytes) can be recruited from tissue into the blood stream across permeable walls of capillaries. Maintaining the critical blood volume ensures that blood volume is maintained relatively constant. Most of patients can recruit fluid at the rate of 500 to 1000 mL/hour. When patients are treated at a faster fluid removal rate, they begin to experience symptomatic hypotension.
Hypotension is the manifestation of hypovolemia or a severe fluid misbalance. Symptomatically, hypotension may be experienced by the patient first as light-headedness. To monitor patients for hypotension, non-invasive blood pressure monitors (NIBP) are commonly used during RRT. When detected early, hypotension resulting from the excessive loss of fluid is easily reversed by giving the patient intravenous fluids. Following administering fluids the RRT operator can adjust the ultrafiltration rate to make the RRT treatment less aggressive. Hct increases in proportion to blood volume loss barring blood loss and can be considered as a surrogate of blood volume. It is known to monitor Hct in the prevention the hypotension by decreasing the ultrafiltrate rate when Hct is seen to increase beyond a desired set value.
The wavelengths of light absorbed by a solute are a characteristic of the solute. Different solutes absorb light at different wavelengths. The concentration of a solute can be determined by measuring the light absorption at the wavelengths corresponding to the solute. If two solutes with different absorption spectra are in solution, their respective concentrations can be determined from the ratio of the light absorbed at two different wavelengths. Hemoglobin (Hb) absorbs less light of wavelength 940 nm (infra red light) than oxyhemoglobin O2Hb, but absorbs more light of wavelength 660 nm (red light)—which is why oxygenated (arterial) blood appears redder than deoxygenated (venous) blood. The four types of hemoglobin have absorption spectra which differ from each other. By using four different wavelengths of light, each corresponding to a type of hemoglobin, the hemoglobin saturation can be determined from the levels of adsorption of light at each of the four wavelength ranges.
In invasive saturation monitoring, light beams only pass through blood and therefore hemoglobin oxygen saturation measurement is easy. The linearity of the Beer-Lambert law may be limited by chemical and instrumental factors, such as: 1) deviations in absorptivity coefficients at high concentrations (>0.01 M) due to electrostatic interactions between molecules in close proximity, 2) scattering of light due to particulates in the sample, 3) fluorescence or phosphorescence of the sample, 4) changes in refractive index at high analyte concentration, 5) shifts in chemical equilibrium as a function of concentration, 6) non-monochromatic radiation, (deviations can be minimized by using a relatively flat part of the absorption spectrum such as the maximum of an absorption band) and 7) stray light.
A sensor for measuring light absorption by a fluid may include a fluid passage, e.g., a blood tube, a light source and light detector defining a light path passing through the fluid passage, and a cuvette that is a structural body for the fluid passage between the light detector and source. It has been discovered that a cuvette body having transparent or reflective walls may improperly transmit light to the light detector. It has been shown that a cuvette body that has light paths not including the blood sample resulting from reflections or refractive body paths known as light pipes will give inaccurate optical transmission readings. It was noticed by the inventors that certain cuvette body materials, e.g., clear tubes, act as light pipes resulting in false transmittance readings. Some of the LED light instead of being transmitted through the liquid medium passed through the cuvette (blood tube) and made its way through the cuvette material to the photodiode. In the case of blood, high Hcts resulted in reflection and scattering with much of the resultant reflected LED lights making their way through the cuvette and to the photodiode via the cuvette wall. This resulted in erroneous transmittance readings which would lead to false Hct or the readings of other substance concentrations and other substance concentrations.
The cuvette blood passage should minimize light scattering and stray light that passes through the fluid passage and reaches the light detector. It has been discovered that clear walls of conventional blood tubes and filtrate tubes transmit reflected light intended to pass directly through the blood. The tubing acts as a light pipe that transmits the reflected light. The light transmitted by the walls of the blood tube may be sensed by the light detector intended to sense light flowing directed from a LED, through the blood cuvette and to the detector. By sensing the reflected light coming from the walls of the blood tube, the light detector may output a false signal that is improperly influenced by the light from the walls of the tube. Extraneous light paths therefore cause errors in measuring transmission and these errors are then propagated into the calculation of absorption. Accordingly, there is a need for a sensor for measuring the light absorption of blood flowing through a tube that does not transmit light into the sensor that has not passed through the blood.